Ceramic cutting tools have been in use for a long time, but the relatively low strength of ceramic materials compared with other common tool materials, such as cemented carbides, and the resulting generally poor performance characteristics of ceramics has limited the application of tools prepared from these materials.
The predominant manufacturing process for the preparation of dense polycrystalline ceramic bodies is hot pressing. In this process, ceramic particles are placed in a graphite mold and sintered under conditions of temperature ranging from 1500.degree. to 1800.degree. C. with a simultaneous pressure application ranging from about 100 to 400 kg./sq.cm. Increases in rupture strength have been obtained through refinement of the grain size of the sintered product. This has required increasingly stringent process conditions encompassing the use of very fine starting powder, utilizing as low a process temperature as would be effective in bringing about the sintering, and the addition of sintering additives. One pronounced advance in alumina-base ceramic cutting tool technology has been the use of alumina-titanium carbide (Al.sub.2 O.sub.3 -TiC) composite material.
Some of the early sintered ceramic cutting tools contained small amounts of carbides, including TiC as sintering additives (Ceramics in Machining Processes., by A. G. King and W. M. Wheildon, Academic Press, New York, 1966), but the first commercial dense polycrystalline ceramic cutting tool containing major amounts of TiC in addition to Al.sub.2 O.sub.3 is a relatively recent development. The production thereof is described in U.S. Pat. No. 3,580,708--Ogawa et al., and employs hot pressing. Experimental production of a similar composition material for hot pressing at higher temperatures (i.e., 1800.degree.-1850.degree. C.) was reported earlier ("Preparation of Alumina-Titanium Carbide Bodies by Hot Pressing Techniques", H. N. Barr, G. D. Cremer and W. J. Koshuba; Powder Met. Bull, Vol. 5, No. 4, September 1950).
Cutting tools containing a significant amount of metal in addition to Al.sub.2 O.sub.3 -TiC are described in U.S. Pat. No. 3,542,529--Bergna et al. The addition of titanium oxide to the Al.sub.2 O.sub.3 -TiC system is described in U.S. Pat. No. 4,063,908--Ogawa et al, such addition making it possible to reduce the hot pressing temperature. All of the above rely upon hot pressing to accomplish densification. Another hot pressing patent is U.S. Pat. No. 4,204,873--Yamamoto et al., in which a different alumina-base system is employed; namely, alumina-tungsten carbide with an addition of titanium nitride.
The sintering of cold pressed powder compacts of aluminum oxide and refractory transition metal diborides is described in U.S. Pat. No. 4,022,584--Rudy. It is also disclosed therein that grain growth stability of the alloy phases is significantly improved by the addition of carbides and nitrides, such additions necessitating higher sintering temperatures or pressure-sintering (i.e., hot pressing). U.S. Pat. No. 4,383,957--Yamakawa et al., describes the sintering of a ceramic composition in an atmosphere of, or containing, carbon monoxide gas. The Yamakawa et al. patent describes hot pressing as having the disadvantage of being "very high priced and unsuitable for the production . . . of an article with a complicated shape" (col. 1, lines 59-62). In the Yamakawa et al. patent, certain sintered bodies were further subjected to hot isostatic pressing to increase the density thereof.
Pending U.S. patent application Ser. No. 332,903--M. Lee and L. Szala, filed Dec. 21, 1981 and assigned to the assignee of the instant invention uses alumina, carbon, and titanium hydride as starting materials, the carbon to titanium ratio being somewhat less than the required ratio for stochiometric TiC.
The use of high heating rates during multi-stage sintering of thoria powder compacts is disclosed in "Material Transport During Sintering of Materials With the Fluorite Structure" by Morgan and Yust [Journal of Nuclear Materials 10, 3 (1963) 182-190, North-Holland Publishing Co., Amsterdam]. Densification data therein for a range of heating rates (i.e. 1.6.degree. to 8.0.degree. C./sec.) shows that the density achieved in compacts of ThO.sub.2 powder heated to a particular temperature and then air quenched was almost independent of the time required to reach that temperature. Data are also reported for heating rates up to 150.degree. C./sec. The maximum theoretical density achieved by their reported techniques was less than 90%.
The following definitions are applicable to an understanding of this invention and/or the prior art:
SINTERING: development of strength and associated densification of a powder compact through the application of heat alone.
HOT PRESSING: the combined application of heat and of pressure applied through the action of a mechanical piston on the powder-filled cavity of a die. Under such conditions the pressure on the powder compact is non-uniformally applied due to die wall friction and the axial application of the piston force. Under proper conditions of temperature and pressure densification of the compact can result.
HOT ISOSTATIC PRESSING (HIP): The simultaneous application of isostatic pressure and heat to a sample body whose porosity is to be reduced. Pressure is applied uniformly to the sample body by an inert gas. The sample body may be (a) a powder compact encapsulated in a gas impermeable, but deformable, envelop such as a tantalum foil can or a glass coating or (b) any solid substantially devoid of open porosity.
ROOM TEMPERATURE: 67.degree.-72.degree. F.
The sintered product of this invention is considered to be "substantially crystalline", because it is not atypical to encounter minor amounts of non-crystalline material (e.g. glasses) in the grain boundary phases.
This invention addresses a particularly troublesome problem encountered in the sintering of multiphase systems. Such systems frequently contain components, which will chemically interact at elevated temperatures. If such chemical reaction proceeds fast enough to inhibit the desired densification or, if the nature of the reaction is such that it results in degradation of the system (i.e. undesirable solid, liquid or gaseous phases are produced), manufacture of the desired product cannot be successfully accomplished by sintering.
This invention is primarily described herein in respect to the Al.sub.2 O.sub.3 -TiC system, because this particular material system presents the very problem in densification discussed herein above. However, the essential aspects of the sintering process disclosed herein are not dependent upon either the use of particular sintering additives, particular material proportions, or the nature of minor impurities. The process is expected to be broadly applicable to the sintering of powdered ceramic materials, that contain components which will chemically react at elevated temperatures to inhibit densification or degrade the system so that an undesirable sintered product results.